1. Technical Field
The present invention relates to a technique that controls the light amount of a light-emitting element, such as an organic light-emitting diode (hereinafter, referred to as ‘OLED’).
2. Related Art
A light-emitting device having a plurality of light-emitting elements arranged therein is used as an exposure head that exposes a photosensitive member so as to form a latent image or a display device that displays various images. The characteristics of such a light-emitting element are being degraded according to a degree of light-emission in the past (for example, the number of times of light-emission) as time passes. In the light-emitting device, the degree of light-emission of each of the light-emitting elements varies according to the shape of an image or a gray-scale level, and thus a variation in characteristic of each light-emitting element (for example, light-emission efficiency) occurs. In particular, when a plurality of images having a common shape are successively output (for example, in an image forming apparatus using a light-emitting device as an exposure head, when the same image is printed in quantity), the variation in characteristic of each light-emitting element is expanding as time passes.
In order to solve the variation in characteristic due to a time-variant degradation of each light-emitting element, for example, JP-A-2003-334990 or JP-A-2002-361924 discloses a technique that causes each light-emitting element to emit light additionally according to the number of times of light-emission of each light-emitting element in the past. According to this technique, the sum of the number of times of light-emission is made uniform, for a plurality of light-emitting elements, and thus the variation in characteristic of each light-emitting element or luminance irregularity can be suppressed.
However, in the technique disclosed in JP-A-2003-334990 or JP-A-2002-361924, in order to hold the number of times of light-emission in each of the plurality of light-emitting elements in the past, a mass storage device is required. Accordingly, the circuit size of the light-emitting device becomes large, and manufacturing costs are increased when the total number of the light-emitting elements or the number of gray-scale levels for a high-definition image, the number of data (the number of times of light-emission) or the number of digits stored in the storage device needs to be increased, and thus the above problem becomes more critical.
An advantage of some aspects of the invention is that it reduces a storage capacity required for suppressing a variation in characteristic of each light-emitting element.
According to a first aspect of the invention, a light-emitting device includes a plurality of light-emitting elements, each emitting light with a light amount according to a driving signal, a storage unit (for example, a storage unit 44 of FIG. 3) that stores first gray-scale data for assigning a gray-scale value of each of the plurality of light-emitting elements, a data processing unit (for example, a data processing unit 45 of FIG. 3) that generates second gray-scale data from the first gray-scale data stored in the storage unit for each light-emitting element such that, as the gray-scale value assigned by the first gray-scale data is large, a gray-scale value assigned by the second gray-scale data is made small, and a driving unit (for example, a driving circuit 30 of FIG. 1) that causes the individual light-emitting elements to emit light in a first period upon supply of a driving signal according to the first gray-scale data stored in the storage unit, and causes the individual light-emitting elements to emit light in a second period different from the first period upon supply of a driving signal according to the second gray-scale data generated by the data processing unit.
According to the first aspect of the invention, the second gray-scale data is generated from the first gray-scale data of each light-emitting element such that the larger the gray-scale value of the first gray-scale data is, the smaller the gray-scale value of the second gray-scale data is. Further, each light-emitting element is driven on the basis of the first gray-scale data in the first period, and is driven on the basis of the second gray-scale data in the second period. According to this configuration, a degree of light-emission of each light-emitting element is made uniform over the plurality of light-emitting elements, as compared with a case where each light-emitting element is driven on the basis of only the first gray-scale data. Therefore, a variation in light amount of each light-emitting element due to a time-variant degradation can be suppressed. Besides, according to the first aspect of the invention, the degrees of light-emission by the first gray-scale data and the second gray-scale data are not necessarily completely matched with each other for each light-emitting element.
Moreover, the light-emitting elements herein are parts for radiating light. More specifically, the light-emitting elements are elements that emit light upon application of electrical energy. The specific structure or material of the light-emitting element herein is arbitrarily selected. For example, an element having electrodes and a light-emitting layer formed of an organic EL material or an inorganic EL material interposed between the electrodes can be used as the light-emitting element of the invention. In addition, various light-emitting elements, such as an LED (Light Emitting Diode) element, an element that emits light by plasma discharge, and so on can be used in the invention Further, the driving signal is specified by, for example, a level (current value or voltage value) and a pulse width (that is, the driving signal has a level component and a pulse width component). ‘The light amount according to driving signal’ herein is a light amount according to the level of the driving signal or a light amount according to the pulse width of the driving signal.
In the invention, ‘such that the larger the gray-scale value assigned by the first gray-scale data is, the smaller the gray-scale value assigned by the second gray-scale data becomes’ means that, paying attention to specified gray-scale values g1a and g1b among all the gray-scale values to be assigned by the first gray-scale data (however, g1a<g1b), a gray-scale value g2a of the second gray-scale data generated from the first gray-scale data having a gray-scale value g1a is larger than a gray-scale value g2b of the second gray-scale data generated from the first gray-scale data having a gray-scale value g1b (g2a>g2b). As regards all the gray-scale values assigned by the first gray-scale data and all the gray-scale values assigned by the second gray-scale data generated by the first-gray scale data, the same relationship is not necessarily established. For example, as described above, when g1b is larger than g1a (‘g1a<g1b’), if the relationship ‘g2a>g2b’ is established, it still falls within the scope of the invention, regardless of the relationship between a certain gray-scale value g1c assigned by the first gray-scale data (≠g1a and g1b) and a gray-scale value g2c of the second gray-scale data generated on the basis of the gray-scale value g1c. 
According to a specific aspect of the invention, the first period (for example, a first period P1 of FIG. 2) is a period where an image (for example, a visual image) according to light-emission by each light-emitting element may be output, and the second period (for example, a second period P2 of FIG. 2) is a period where the image according to light-emission by each light-emitting element is not output. According to this configuration, light emission in the second period does not have an effect on the visual image to be formed in the first period, and thus a desired image can be formed with high quality.
According to a specific aspect of the invention, the second period may be shorter than the first period. According to this configuration, as compared with the configuration that the first period and the second period have the same time length, a time length that can be originally used to form an image in the first period can be secured relatively long. Therefore, the image can be efficiently formed.
In the above aspects, a specific configuration for making the second period shorter than the first period is arbitrarily selected. For example, the total number of the light-emitting elements that actually emit light in the second period may be made smaller than the total number of the light-emitting elements that emit light in the first period, and thus the second period may have a time length shorter than the first period. However, according to a preferred aspect of the invention, the driving signal supplied to each light-emitting element may become a level (current value or voltage value) for causing the light-emitting element to emit light by a pulse width according to the first gray-scale data of a first unit period (for example, a unit period U1 of FIG. 2 or FIG. 4) in the first period, and may become a level for causing the light-emitting element to emit light by a pulse width according to the second gray-scale data of a second unit period (for example, a unit period U2 of FIG. 2 or FIG. 4) shorter than the first unit period in the second period. That is, a pulse width (for example, a pulse width Wb of FIG. 7) of the driving signal having a predetermined gray-scale value assigned by the second gray-scale data is shorter than a pulse width (for example, a pulse width Wa of FIG. 7) of the driving signal having the predetermined gray-scale value assigned by the first gray-scale data. According to this configuration, since the second unit period is set to have the time length shorter than the first unit period, for example, when all the light-emitting elements that are driven in the first period are driven in the second period, the second period can be reliably made shorter than the first period.
However, if the level of the driving signal is set to the first period and the second period, and the second unit period is set to have a time length shorter than the first period, there is a possibility that degrees of light-emission of each light-emitting element in the first period and the second period are different from each other. According to a preferred aspect of the invention, the driving signal supplied to each light-emitting element may become a level (for example, an on current value Ia of FIG. 7) for causing the light-emitting element to emit light with a first light amount (for example, an intensity La of FIG. 7) in a pulse width according to the first gray-scale data of the first unit period, and may become a level (for example, an on current value Ib of FIG. 7) for causing the light-emitting element to emit light with a second light amount (for example, intensity Lb of FIG. 7) larger than the first amount in a pulse width according to the second gray-scale data of the second unit period. According to this configuration, the degree of light-emission of each light-emitting element can be made uniform over the first period and the second period with high accuracy.
By the way, among the light-emitting elements, there may be an element that has different states of the time-variant change in characteristic when the level of the driving signal is fixed and the pulse width is changed, and when the pulse width of the driving signal is fixed and the level is changed. In the light-emitting device that uses such a light-emitting element, the pulse width and the level of the driving signal according to the second gray-scale data are set such that the state of the time-variant change in characteristic of the light-emitting element when the driving signal according to the second gray-scale data assigning a predetermined gray-scale value may be supplied approximately matches with the state of the time-variant change in characteristic of the light-emitting element when the driving signal according to the first gray-scale data assigning the predetermined gray-scale value is supplied. According to this configuration, a speed of time-variant characteristic degradation of each light-emitting element can be made uniform for the plurality of light-emitting elements.
Moreover, ‘the state of the time-variant change in characteristic of the light-emitting element’ means the relationship between a time elapsed from a time point at which the light-emitting element is produced (or a time elapsed from a time point at which the use of the light-emitting device starts) and the characteristic of the light-emitting element. In general, it is a characteristic change speed of the light-emitting element. Further, lifespan representing a time until a characteristic value (for example, a light amount when a predetermined gray-scale level is assigned) of the light-emitting element is lowered to a predetermined value corresponds to the state of the change in characteristic of the light-emitting element in the invention. The characteristic of the light-emitting element includes, for example, a light amount of the light-emitting element when a predetermined gray-scale value is assigned or a relative ratio (light-emission efficiency) between the value of a current supplied to the light-emitting element and a light amount at that time.
For example, a time-variant lowering speed of the light amount of each light-emitting element, such as an OLED element, may be approximately in proportion to the pulse width of the driving signal and to an M power (where M is a real number) of the level of the driving signal. In the configuration that uses such a light-emitting element, the light-emitting element having a predetermined gray-scale value assigned by the first gray-scale data emits light with a light amount La upon supply of a driving signal having a pulse width Wa, the level of the driving signal according to the second gray-scale data may be determined such that a light amount Lb of the light-emitting element to which a driving signal having a pulse width Wa/u (where u>1) according to the second gray-scale data assigning the predetermined gray-scale value satisfies the equation Lb/La=u1/M. Alternatively, when the light-emitting element having a predetermined gray-scale value assigned by the first gray-scale data emits light with a light amount La upon supply of a driving signal having a pulse width Wa, a pulse width Wb of a driving signal that has a level determined so as to cause the light-emitting element to emit light with a light amount La×v (where v>1) according to the second gray-scale data assigning the predetermined gray-scale value may satisfy the equation Wb/Wa=v−M.
The light-emitting device according to the aspect of the invention is used in various electronic apparatuses. As the electronic apparatus, there is an image forming apparatus that has the light-emitting device according to the aspect of the invention as an exposure device (an exposure head). The image forming apparatus includes an image carrier having an image formation surface, on which a latent image is formed on an image formation surface by exposure, the light-emitting device according to the aspect of the invention that exposes the image formation surface, and a developing device that attaches a developing agent (for example, a toner) to the latent image so as to form an apparent image. In the light-emitting device according to the aspect of the invention, irregularity of the light amount (gray-scale level) of each light-emitting element can be suppressed for a long period. Therefore, according to the image forming apparatus using the light-emitting device, uniform-quality images can be formed on recording mediums for a long period.
According to a specific aspect of the image forming apparatus, the first period may be a period where a developing device forms an apparent image from a latent image formed on an image carrier by light-emission of each light-emitting element in that period, and the second period may be a period, a gap between previous and next first periods, where the apparent image according to light-emission of each light-emitting element is not formed in that period. According to this configuration, since light-emission in the second period does not have an effect on the visual image to be formed in the first period, a desired image can be formed with high quality. Moreover, a configuration for causing the visual image (apparent image) according to light-emission by each light-emitting element not to be formed in the second period is arbitrarily selected. For example, a configuration for causing a developing agent not to be attached to a latent image formed on a photosensitive member by light-emission of each light-emitting element in the second period may be selected. Alternatively, a configuration for causing the latent image not to be formed on the photosensitive member by light-emission of each light-emitting element in the second period (for example, causing the photosensitive member not to be charged in the second period) may be selected.
Besides, the use of the light-emitting device according to the invention is not limited to exposure. For example, the light-emitting device according to the aspect of the invention can be used as display devices of various electronic apparatuses. Such an electronic apparatus includes, for example, a personal computer or a cellular phone. Further, the light-emitting device according to the aspect of the invention is suitably used as various illumination devices, such as a device that is disposed at the back of a liquid crystal device and illuminates the liquid crystal device (backlight) or a device that is mounted on an image reading apparatus, such as a scanner or the like, and irradiates light onto an original.
Another aspect of the invention is also specified as a circuit for driving a light-emitting device (a driving circuit 30 and a controller 40 of FIG. 1). A driving circuit for a light-emitting device, which has a plurality of light-emitting elements, each emitting light with a light amount according to a driving signal, includes a storage unit that stores first gray-scale data for assigning a gray-scale value of each of the plurality of light-emitting elements, a data processing unit that generates second gray-scale data from the first gray-scale data stored in the storage unit for each light-emitting element such that the larger the gray-scale value assigned by the first gray-scale data is, the smaller a gray-scale value assigned by the second gray-scale data becomes, and a driving unit that causes each light-emitting element to emit light upon supply of a driving signal according to the first gray-scale data stored in the storage unfit in a first period, and causes each light-emitting element to emit light upon supply of a driving signal according to the second gray-scale data generated by the data processing unit in a second period different from the first period. According to this driving circuit, a storage capacity of the storage unit can be reduced, and a variation in characteristic of each light-emitting element can be suppressed.
Another aspect of the invention is also specified as a method of driving a light-emitting device. A method of driving a light-emitting device, which has a plurality of light-emitting elements each emitting light with a light amount according to a driving signal, includes acquiring first gray-scale data assigning a gray-scale value of each of the plurality of light-emitting elements, generating second gray-scale data from the acquired first gray-scale data for each light-emitting element such that, as the gray-scale value assigned by the first gray-scale data is large, the gray-scale value assigned by the second gray-scale data is made small, and causing each light-emitting element to emit light upon supply of a driving signal according to the acquired first gray-scale data in a first period, and causing each light-emitting element to emit light upon supply of a driving signal according to the generated second gray-scale data in a second period different from the first period. According to this driving method, the same effects as those in the light-emitting device according to the aspect of the invention can be obtained.